Patients with Multiple Endocrine Neoplasia Type 2 (MEN2) exhibit a series of oncogenic events including metastatic medullary thyroid carcinoma (MTC), pheochromocytomas, ganglioneuromas, parathyroid adenomas, and mucosal neuromas;MEN2B patients can also display developmental defects. Most patients diagnosed with MEN2 contain one of several dominantly-acting point mutations in the Ret receptor tyrosine kinase, resulting in ligand-independent activation of the receptor. Similar effects have been observed with sporadic MTCs, which often contain Ret mutations. This Proposal seeks to build on a series of experiments utilizing the Drosophila eye as a model system for MEN2. The Drosophila eye was chosen due to the availability of powerful molecular and genetic tools. Orthologous mutations were engineered into the Drosophila Ret ortholog dRet;targeting expression of these dRetMEN2 isoforms to the eye resulted in hyperproliferation and developmental defects that were strikingly reminiscent of symptoms observed in MEN2 and sporadic MTC patients. Genetic screens identified 138 functional, genetic modifiers of the dRetMEN2 phenotype. These modifiers fell into a small number of signal transduction pathways as well as other classes. One goal of the current Proposal is to characterize the role of selected loci in the proliferative response. Human orthologs of ten of these modifiers (genetic enhancers) were analyzed in pheochromocytomas and MTCs in an effort to identify susceptibility loci;two loci have merited direct sequencing of their open reading frames. In addition, a new approach has been developed that optimizes the use of these Drosophila models in high throughput assays dedicated to identifying lead therapeutic compounds. Recent success with one such compound has led to clinical trials for MEN2 patients, providing proof-of-concept that this approach can be successful. Relevance to public health: While important advances continue to be made in the detection and treatment of cancer, the overall death rates have come down incrementally. This Proposal uses the fruitfly as a simple model both to understand the genes involved in directing the cancer syndrome Multiple Endocrine Neoplasia Type 2. These same flies will be used in a novel approach to directly identify drugs to treat this syndrome.